[GME]

To test out if the extrusions under the rails will help me get things right, I am going to design some parts I can 3d print out of polycarbonate blend I have on hand. If they work well enough I may just leave them in place, also if they work well I will post up a link to the file for others to print or have printed. I'm trying to decide how extensive I want to make them...for example making them so you can adjust "toe in / toe out" as far as flatness goes. On my last and final attempt to parallel the rails, I had one rail tightened down and it rolled very smoothly with the other rail loose...when I tried the same test with the rails reversed (swapped tight and loose rails) the action was back to being with much more drag...seemed odd to me.

Erik,

Yes, the results of your test seem odd. Sounds to me like one or both of the rails isn't perfectly straight. Not that odd, though. I've searched for, but never found, a manufacturing tolerance for straightness. I also seem to recall one vendor commenting on its website that the rails may not be perfectly straight 'out of the box,' so to speak. I believe that is why having a milled reference edge is preferred for rail mounting. You can clamp the rail to the edge and tighten it down. I used custom jigs and a precision straight edge when I set mine, but I had a relatively flat surface to work from. Apparently, you don't.

I do not believe anything you can make on a 3d printer is going to be up to the task of making an adequate linear rail surface. IMO, printed parts may be fine for making prototypes for testing, but not for a solution. I can't imagine plastic being even close to rigid enough. Also, not much strength for screw holding and threads holding up under vibration. However, making prototypes for testing purposes seems like a perfect diagnostic approach and modeling for a permanent solution.

Here's a reference source that includes information for mounting linear rails. You may find it useful. https://www.hiwin.com/pdf/linear_guideways.pdf

Gary